In the heterogeneous reaction of an oil-soluble component with a water-soluble component, a stirring method using power, an emulsification method using surface active agents, a critical reaction method under high-temperature and high-pressure conditions, and so forth have heretofore been employed in order to obtain uniform mixing of the reactants. These methods, however, have various disadvantages, such as consumption of significant energy for the stirring operation or for maintaining high-temperature and high-pressure conditions, danger involved in realizing the high-temperature and high-pressure conditions, and a need to break emulsions after the completion of the reaction.